The primary purpose of this project is to elucidate the mechanism of action of fluoxetine in premenstrual dysphoric disorder (PMDD) by ascertaining whether fluoxetine alters brain neurosteroid levels as measured in cerebrospinal fluid (CSF). A secondary objective is to find out whether the increase in neurosteroid levels in plasma parallets that in the brain, as inferred from the CSF measurements. A third objective is to investigate the role of PS, a compound which antagonizes activiey at the GABA-A receptor.